Optical system for beam shaping

ABSTRACT

An optical system for shaping a laser beam includes a beam shaping element configured to receive the laser beam having a transverse input intensity profile and to impose a beam shaping phase distribution onto the laser beam. The optical system further includes a near field optical element, arranged downstream of the beam shaping element at a beam shaping distance and is configured to focus the laser beam into the focus zone. The imposed phase distribution results in a virtual optical image of the elongated focus zone located before the beam shaping element. The beam shaping distance corresponds to a propagation length of the laser beam within which the imposed phase distribution transforms the transverse input intensity profile into a transverse output intensity profile at the near field optical element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority under 35U.S.C. § 120 to PCT Application No. PCT/EP2015/076707 filed on Nov. 16,2015, which claims priority to German Application No. 10 2014 116 957.3,filed on Nov. 19, 2014. The entire contents of these priorityapplications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an optical system for beam shaping alaser beam and in particular for beam shaping a laser beam forprocessing materials that are essentially transparent for the laserbeam. Moreover, the invention relates to a method for beam shaping.

BACKGROUND

There are many possibilities for using absorption of light forprocessing a workpiece, in particular by introducing localizedmodifications into the workpiece. The so-called volume absorption, i.e.,an absorption that is not limited to the surface, opens the possibilityto process brittle-hard materials that are essentially transparent forthe laser beam. Generally, volume absorption benefits from a kind ofnonlinear absorption, at which an interaction with the material takesplace only at a material dependent (threshold) intensity.

SUMMARY

Herein, a nonlinear absorption is understood as an intensity dependentabsorption of light, that is not primarily based on the directabsorption of the light. Instead, it is based on an increase of theabsorption during interaction with the incident light, often atemporally limited laser pulse. Thereby, electrons can absorb that muchenergy by inverse bremsstrahlung that further electrons are set free byimpacts, so that the rate of generating electrons overcomes that rate ofrecombination. Under specific conditions, those initial electrons, whichare required for the avalanche-like absorption, may already be presentfrom the start or may be generated by an existing rest-absorption bylinear absorption. For example, for ns-laser pulses, an initialionization may result in an increase in temperature that causes anincrease of the number of free electrons and therefore of the followingabsorption. Under other conditions, such initial electrons may begenerated by multi-photon ionization or tunnel ionization as examples ofwell-known nonlinear absorption mechanisms. For ultrashort laser pulseswith, for example, sub-ns-pulse durations such an avalanche-likegeneration of electrons can be utilized.

A volume absorption may be used for materials, which are essentiallytransparent for the laser beam (herein in short referred to astransparent materials), for forming a modification of the material in anelongated focus zone. Such modifications may allow separating, drilling,or structuring of the material. For separating, for example, rows ofmodifications may be generated that cause a breaking within or along themodifications. Moreover, it is known to generate modifications forseparating, drilling, and structuring that allow a selective etching ofthe modified areas (SLE: selective laser etching).

The generation of an elongated focus zone can be affected with the helpof apodized Bessel beams (herein also referred to as quasi-Bessel beam).Such beam profiles may be formed, for example, with an axicon or aspatial light modulator (SLM: spatial light modulator) and an incidentlight beam having a Gaussian beam profile. A subsequent imaging into atransparent workpiece results in the intensities required for volumeabsorption. Quasi-Bessel beams—like Bessel beams—usually have aring-shaped intensity distribution in the far field of the beam profileexisting within the workpiece. Calculating phase distributions for beamshaping quasi-Bessel beams, e.g., with an SLM is disclosed in Leach etal., “Generation of achromatic Bessel beams using a compensated spatiallight modulator,” Opt. Express 14, 5581-5587 (2006), incorporated hereinby reference in its entirety.

Moreover, systems are known for forming a line of intensityenhancements, e.g., with the help of multifocal lenses. Thereby, a phasemodification of the laser beam to be focused is per-formed in the farfield, i.e. during focusing, whereby the phase modification results inthe formation of longitudinally displaced focus zones.

An aspect of the present disclosure has the objective to provide anoptical system that enables beam shaping for a tailored volumeabsorption. In particular, the objective is, for laser processingapplications, to provide in beam propagation direction elongated,slender beam profiles with a high aspect ratio for processingtransparent materials.

At least one of the objectives is solved by an optical system of claim1, a laser processing ma-chine of claim 12, a method for beam shaping alaser beam of claim 15, and a method for laser material processing ofclaim 17. Further developments are given in the dependent claims.

In an aspect, there is disclosed an optical system for beam shaping of alaser beam for processing an in particular transparent material bymodifying the material in a focus zone being elongated in propagationdirection. The optical system includes a beam shaping element that isconfigured to receive the laser beam having a transverse input intensityprofile and to impose a beam shaping phase distribution over thetransverse input intensity profile onto the laser beam. In addition, theoptical system includes a near field optics located downstream of thebeam shaping element at a beam shaping distance and configured to focusthe laser beam into the focus zone. Thereby, that imposed phasedistribution is such that a virtual optical image of the elongated focuszone is attributed to the laser beam, the optical image being before thebeam shaping element, and the beam shaping distance corresponds to apropagation length of the laser beam within which the imposed phasedistribution transforms the transverse input intensity profile into atransverse output intensity profile in the region of the near fieldoptics, wherein the output intensity profile has, in comparison with theinput intensity profile, a local maximum positioned outside of the beamaxis.

In a further aspect, an optical system is disclosed for beam shaping alaser beam for processing an in particular transparent material bymodifying the material. The optical system includes a beam shapingelement for imposing a phase distribution of an inverse quasi-Besselbeam (e.g., inverse quasi-Bessel like beam) profile and/or of an inversequasi-Airy beam (e.g., inverse quasi-Airy like beam) profile onto thelaser beam, and a near field optics for focusing the phase imposed beam.The phase distribution is selected such that the focusing of the phaseimposed beam forms an inverse quasi-Bessel beam profile and/or aninverse quasi-Airy beam profile having an, in propagation direction ofthe laser beam elongated, focus zone, at which only a central region ofthe incident laser beam makes contributions to a downstream end of theelongated focus zone.

In a further aspect, a laser processing machine for processing atransparent material with a laser beam by modifying the material withina focus zone, which is elongated in the propagation direction of thelaser beam, includes a laser beam source, such an optical system, and aworkpiece positioning unit for positioning the material as the workpieceto be processed.

In a further aspect, a method is disclosed for beam shaping of a laserbeam with a transverse input intensity profile for processing of an inparticular transparent material by modifying the material in an, inpropagation direction elongated, focus zone. The method includes thestep of imposing a beam shaping phase distribution onto the transverseinput intensity profile, wherein the imposed phase distribution is suchthat a virtual optical image of the elongated focus zone is attributedto the laser beam. Moreover, the method includes the step of propagatingthe laser beam over a beam shaping distance, after which the imposedphase distribution has transferred the transverse input intensityprofile into a transverse output intensity profile, so that thetransverse output intensity profile in comparison to the input intensityprofile includes a local maximum positioned outside of the beam axis.Moreover, the method includes the step of focusing the laser beam intothe focus zone for forming a near field based on the output intensityprofile.

In a further aspect, a method is disclosed for laser material processingof an in particular trans-parent material by modifying the material witha laser beam, wherein the method includes the following steps:generating an inverse quasi-Bessel laser beam profile and/or a laserbeam profile of an inverse accelerated beam, herein also referred to asa quasi-Airy beam-like laser beam profile, with an in propagationdirection elongated focus zone by phase-modulation of the laser beam,and positioning the elongated focus zone at least partly in the materialto be processed.

In a further aspect, the use of an inverse quasi-Bessel beam profileand/or of an inverse quasi-Airy beam profile for laser materialprocessing of an in particular transparent material by modifying thematerial within an elongated focus zone of the inverse quasi-Bessel beamprofile and/or of the inverse quasi-Airy beam profile is dis-closed.Thereby, an inverse quasi-Bessel beam profile and/or an inversequasi-Airy beam profile can be characterized, for example, by one ormore of those features, which are disclosed herein as characterizing, inparticular by the attribution of a virtual image before the beam shapingelement, by the, in comparison with respective conventional beamsinverted, radial distributions of amplitude/intensity, and by the ingeneral fixed position of the end of the focus zone.

Herein, concepts are disclosed that allow to at least partly improveaspects of the prior art. In particular additional features and theirfunctionalisms result from the following description of embodiments onthe basis of the drawings. The drawings show:

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of an optical system for beam shapingof a laser beam;

FIG. 2 is a schematic illustration of a laser processing device with anoptical system ac-cording to FIG. 1 for material processing;

FIG. 3 is a schematic illustration of an optical system for explainingthe optical functioning;

FIG. 4 is an example of a longitudinal intensity distribution in anelongated focus zone after imaging a virtual optical image;

FIG. 5 is a ZR-section of the longitudinal intensity distribution shownin FIG. 4;

FIG. 6 is an exemplary experimental study on the modification of atransparent material in an elongated focus zone according to FIGS. 4 and5;

FIG. 7 is a schematic illustration for explaining the generation andimaging of a real intensity enhancement,

FIG. 8 is an example of a longitudinal intensity distribution in anelongated focus zone after imaging a real intensity enhancementaccording to FIG. 7;

FIG. 9 is a schematic illustration of a first example of an opticalsystem based on a hollow cone axicon;

FIG. 10 is a schematic illustration of a second example of an opticalsystem based on a hollow cone axicon;

FIG. 11A and FIG. 11B are schematic illustrations of examples foroptical systems based on a reflective axicon;

FIG. 12 is a schematic illustration of an example of an optical systembased on a spatial light modulator;

FIG. 13 is a schematic illustration of an example of an optical systembased on a transmit-ting diffractive optical element;

FIG. 14 is a schematic illustration of an example of a phasedistribution in a diffractive optical element in an optical systemaccording to FIG. 13;

FIG. 15 is an exemplary intensity cross-section of an output intensityprofile in an optical system according to FIG. 13;

FIG. 16 is an XY-view of the output intensity profile of the intensitycross-section shown in FIG. 15;

FIG. 17 is a schematic illustration of an example of an optical systemwith filtering non-phase-modulated beam portions;

FIG. 18 is a schematic illustration of an example of an optical systembased on a diffractive optical element with a linear phase contributionfor separating a phase-modulated beam portion;

FIG. 19 is a schematic illustration of an example of an optical systemwith a scan device;

FIG. 20 is a schematic illustration for explaining the imaging system ofan optical system;

FIG. 21 is a schematic illustration for explaining an optical system forthe incidence of a converging laser beam;

FIG. 22 is a schematic illustration for explaining an optical systemwith adaptation of the divergence;

FIG. 23 is an exemplary cross-section of the intensity of an outputintensity profile in an optical system for generation of a flat-topintensity profile;

FIG. 24 is an XY-view of the output intensity profile of the intensitycross-section shown in FIG. 23;

FIG. 25 is an example of a longitudinal intensity distribution thatresults from the output intensity profile of FIGS. 23 and 24;

FIG. 26 is an exemplary experimental study on the modification of atransparent material in an elongated focus zone according to FIG. 25;

FIG. 27 is an example of a longitudinal intensity distribution whenusing a multifocal near field optics;

FIG. 28 is a schematic illustration of an example of a phasedistribution for generating an inverse Airy beam shape with adiffractive optical element for use in an optical system according toFIG. 13;

FIG. 29 is an exemplary intensity cross-section of an output intensityprofile for generating the inverse Airy beam shape according to FIG. 28;

FIG. 30 is an XY-view of the output intensity profile of the intensitycross-section shown in FIG. 29;

FIG. 31 is an example of a longitudinal intensity distribution in anelongated focus zone for the inverse Airy beam shape generated with thephase distribution according to FIG. 28;

FIG. 32 is a schematic illustration for explaining the imaging of avirtual image in combi-nation with the imaging of a real intensityenhancement;

FIG. 33A to FIG. 33D are beam profiles for an inverse quasi-Bessel beamat the propagation from the beam shaping element to the near fieldoptics; and

FIG. 34 is an amplitude distribution for a section along the beam axis Zfor illustration of the positions of the beam profiles of FIGS. 33Athrough 33D.

DETAILED DESCRIPTION

Herein described aspects are based partly on the realization that, dueto the high intensities needed for laser processing, intensities may bepresent already during the preparation of the laser beam that result indamage of optical elements. In view thereof, it was further realizedthat the generation of an elongated focus zone within the workpiece maybe based on the imaging of a virtual beam profile. By this concept ofimaging a virtual beam profile, regions with intensity peaks can bereduced or even avoided in the optical system. It was further realizedthat a phase distribution attributed to the virtual beam profile may beimposed onto the laser beam that causes the desired change of theintensity distribution in the far field. In particular, it was realizedthat by a far field distribution, which originates from such a virtualbeam profile, for example, inverse-Bessel beam-like or inversequasi-Airy beam-like intensity distributions, specifically designedintensity distributions, and in particular superpositions of the same inthe focus zone can be created. For such intensity distributions, alateral energy entry into the focus zone can take place, which inparticular enables the processing of transparent materials. It wasfurther realized that, in comparison to systems for imaging a realintensity enhancement, the concept of the imaging of a virtual beamprofile may lead to shorter configurations of such optical systems.

An elongated focus zone relates herein to a three-dimensional intensitydistribution defined by the optical system that determines the spatialextent of the interaction and thereby the modification within thematerial to be processed. The elongated focus zone determines thereby anelongated region in which a fluence (energy per area)/intensity ispresent within the material to be processed, which is beyond thethreshold fluence/intensity being relevant for theprocessing/modification. Usually, one refers to elongated focus zones ifthe three-dimensional intensity distribution with respect to a targetthreshold intensity is characterized by an aspect ratio (extent inpropagation direction in relation to the lateral extent) of at least10:1, for example 20:1 and more, or 30:1 and more. Such an elongatedfocus zone can result in a modification of the material with a similaraspect ratio. In some embodiments, focus zones can be formed that are,for example, also in propagation direction parallel with respect to eachother, wherein each of the focus zones has a respective aspect ratio. Ingeneral, for such aspect ratios, a maximal change of the lateral extentof the (effective) intensity distribution over the focus zone can be inthe range of 50% and less, for example 20% and less, for example in therange of 10% and less.

Thereby, the energy within an elongated focus zone can be laterallysupplied essentially over the complete length of the createdmodification. As a consequence, a modification of the material in theinitial region of the modification does not have or hardly has anyshielding effects on the part of the laser beam that causes amodification of the material downstream of the beam, for example, in theend region of the modification zone. In that sense, a Gaussian beamcannot generate a comparable elongated focus, because the energy supplyis performed essentially longitudinally and not laterally.

The transparency of a material, which is essentially transparent for alaser beam, relates herein to the linear absorption. For light below thethreshold fluence/intensity, material, which is essentially transparentfor a laser beam, may absorb, for example, along a length up to the backend of the modification, e.g., less than 20% or even less than 10% ofthe incident light.

Herein described aspects further are partly based on the realizationthat by a desired beam shaping, for example, with a diffractive opticalelement (DOE), the density of free electrons, which is created in thematerial by nonlinear absorption, may be tailored. Along the therebycreated modifications, a crack formation may be specifically guided,which then results in the separation of the workpiece.

Herein described aspects further are based partly on the realizationthat, for a DOE, multiple phase distributions can be provided in thephase distribution of a phase mask, for example, in respective segments.Thereby, in particular the advantages of the concept of a virtualoptical image, for example, an inverse quasi-Bessel beam shape, can beused at the superposition of the imaging of multiple such virtual images(in longitudinal or lateral direction), wherein also the interaction(e.g. interference) and spatial constellation of multiple imaging mayhave effects onto the formation of the common focus zone. In addition,it was recognized that in this manner asymmetric ‘common’ focus zonescan be created. For example, for material processing, asymmetric‘common’ focus zones create a preference for a specific movementdirection or a specific separation direction. Moreover, it wasrecognized that, during the laser processing, such preferred directionsmay be adopted to desired processing trajectories by orienting/turningthe DOE within an optical system. For digital phase masks (SLMs etc.), adirect controlling of the phase distribution may further be performed toadapt the preferred direction.

Herein described aspects further are based in part on the realizationthat, by the use of a DOE, additional phase distributions may be imposedonto the beam, which, for example, may simplify the setup of theunderlying optical systems and/or the isolation of a usable beamportion.

In other words, disadvantages of the prior art may in some embodimentsat least partly be overcome by an optic concept, in which the beamprofile, which is positioned in the region of the workpiece and which iselongated in propagation direction, is affected by an imaging of acreated virtual beam profile. In some embodiments, the optic conceptfurther allows a filtering possibility for undesired beam portions, forexample, in a region of the Fourier-plane of the beam profile and aseparation of the beam shaping from the focusing.

The systems and methods resulting from these realizations can inter aliaenable separating of transparent, brittle-hard materials with highvelocity and with good quality of the cutting edge. Moreover, suchsystems and methods may further enable separating without a taper angleas it is created in ablating methods. In particular when separatingbased on non-ablating modifications, there may be no or only a smallremoval, with the consequence that the material has only a few particleson the surface after the processing.

In the following, the underlying optical concept will be generallyexplained with reference to FIGS. 1 to 8. Then, exemplary embodiments ofoptical systems will be explained, which, on the one side, implement theoptical system by conventional optics such as lenses and mirrors (seeFIGS. 9 to 11B) and, on the other side, by diffractive optical elements(see FIGS. 12 to 16). In connection with FIGS. 17 to 22, thecombinability of the optical system with components and aspects forfiltering and scanning as well as general aspects of the beamdevelopment within the optical system are explained. Finally, inconnection with FIGS. 23 to 32, exemplary embodiments of the elongatedfocus zones for material processing are illustrated, which in particularcan be realized with diffractive optical elements. In FIGS. 33A to 33Dand 34, beam profiles and a longitudinal amplitude distribution areexplained for an inverse quasi-Bessel beam at the propagation from thebeam shaping element to the near field optics in the optical system.

FIG. 1 shows a schematic illustration of an optical system 1 for beamshaping a laser beam 3 with the aim to create a focus zone 7, which iselongated in a propagation direction 5, within a material 9 to beprocessed. Generally, laser beam 3 is determined by beam parameters suchas wavelength, spectral width, temporal pulse shape, formation of pulsegroups, beam diameter, transverse input intensity profile, transverseinput phase profile, input divergence, and/or polarization. According toFIG. 1, laser beam 3 is supplied to optical system 1 for beam shaping,i.e., for transforming one or more of the beam parameters. Usually, forlaser material processing, laser beam 3 will be a collimated Gaussianbeam with a transverse Gaussian intensity profile, which is generated bya laser beam source 11, for example a ultrashort pulse high-intensitylaser system. The transformation can be performed, for example, into aninverse Bessel beam-like or inverse Airy beam shape.

In the laser processing machine 21 shown in FIG. 2, optical system 1may, for example, be used for material processing. Laser processingmachine 21 includes a support system 23 and a workpiece positioning unit25. Support system 23 spans over workpiece positioning unit 25 andcarries laser system 11, which is integrated in FIG. 2, for example, inan upper crossbeam 23A of support system 23. In addition, optical system1 is mounted at crossbeam 23A to be displaceable in X direction, so thatboth components are arranged close to each other. In alternativeembodiments, laser system 11 may be provided, for example, as a separateexternal unit. Laser beam 3 of laser system 11 is guided to opticalsystem 1 by optical fibers or as a free propagating beam.

Workpiece positioning unit 25 carries a workpiece that extends in theX-Y-plane. The work-piece is the material 9 to be processed. Forexample, the material to be processed includes a glass plate or a platein ceramic or crystalline embodiment such as sapphire or silicon, thatis essentially transparent for the laser wave-length used. Workpiecepositioning unit 25 allows displacing the workpiece in Y directionrelative to support system 23, so that, in combination with thedisplaceability of optical system 1, a processing area is provided,which extends within the X-Y-plane.

According to FIG. 2, in addition, optical system 1 or cross-beam 23A isrelocatable in Z direction, such that the distance to the workpiece canbe set. For a cut running in Z direction, the laser beam is usually alsodirected in the Z direction (i.e., normal to the workpiece) onto theworkpiece. However, additional processing axes may be provided asexemplarily illustrated in FIG. 2 by the boom arrangement 27 and theadditional rotational axes 29. Accordingly, boom arrangement 27 is anoptional in the embodiment of FIG. 2. In addition, redundant add-on axesmay be provided for higher dynamics, as, for example, not the workpieceor the optical system, but more compact and respectively designedcomponents are accelerated.

Laser processing machine 21 further includes a control unit notexplicitly shown in FIG. 1, which is, for example, integrated withinsupport system 23 and which in particular includes an interface forinputting operation parameters by a user. In general, the control unitincludes elements for controlling electrical, mechanical, or opticalcomponents of laser processing ma-chine 21, for example, by controllingrespective operation parameters such as pump laser power, cooling power,direction and velocity of the laser machine and/or the workpiecepositioning unit, electrical parameters for setting an optical element(for example, of an SLM) and the spatial orientation of an opticalelement (for example, for rotation of the same).

Additional arrangements for laser processing machines with variousdegrees of freedom are disclosed, for example, in EP 1 688 807 A1,incorporated herein by reference in its entirety. In general, forsmaller workpieces often only the workpiece is moved, and for largerworkpieces only the laser beam or—as in FIG. 2—the work-piece and thelaser beam are moved. Moreover, two or more optical systems and, thus,focus zones may be supplied by a single laser system 11.

The modifications within the material, which are generated by the laserprocessing machine, may be used, for example, for drilling, separatingby induced tensions, welding, creating a modification of the refractionbehavior, or for selective laser etching. Accordingly, it is importantto control the geometry as well as the type of modification in asuitable manner. Besides parameters such as laser wavelength, temporalpulse shape, number of pulses, energy and temporal distance of thepulses within a pulse group creating an individual modification, as wellas pulse energy or pulse group energy, the beam shape plays a decisiverole.

In particular, an elongated volume modification allows processing of a,in beam propagation direction, volume region within a single processingstep. In particular, at one position in feed direction, the processingcan take place over a large extent in only a single modificationprocessing step. By the use of the optical systems, beam shapes, andmethods described herein, one can achieve, on the one side, better workresults (in comparison to single modifications that are positioned nextto each other at one position in feed direction in succeedingmodification processing steps) and, on the other side, one can reducethe processing time and the requirements for the system technology.Then, for single modifications, multiple working steps are needed thatincrease the time needed and that require a more involved ensuring ofrelative positions of the single modifications.

In addition, an elongated focus zone can be helpful when processinguneven materials, because essentially identical laser processingconditions are given along the elongated focus zone such that, in thoseembodiments, a respective readjusting in propagation direction may notbe necessary or only be necessary starting at a larger deviation of theposition of the material to be processed than the lengths of theelongated focus area (in consideration of the requiredprocessing/intrusion depth).

In general, it applies to the processing of transparent materials byelongated volume absorption that, as soon as absorption takes place,that absorption itself or the resulting changes in the materialproperties can influence the propagation of the laser beam. Therefore,it is advantageous, if beam portions, which should cause a modificationdeeper within the workpiece, i.e., in beam propagation directiondownstream, essentially propagate not through regions of considerableabsorption.

In other words, it is favorable to lead those beam portions, whichcontribute to the modification further downstream, under an angle to theinteraction zone. An example for this is the quasi-Bessel beam, forwhich a ring-shaped far-field distribution is given, the ring width ofwhich is typically small in comparison to the radius. Thereby, the beamportions of the interaction zone are led in essentially with that anglein rotational symmetry. The same applies for the inverse quasi-Besselbeam or for modifications or extensions of the same such as thehomogenized or modulated inverse quasi-Bessel beam described herein.Another example is the inverse accelerated ‘quasi-Airy beam-like’ beam,for which the beam portions are led into the modification under anoffset angle, where this is done clearly tangential and—not as for thepure quasi-Bessel beam rotationally symmetric—to the curved modificationzone, e.g. as for a curved inverse quasi-Bessel beam.

Moreover, it is desired to considerably pass the threshold for thenonlinear absorption only within the desired volume region and to choosethe geometry of that volume area such that it is suitable for thedesired application, but that also the propagation to further downstreampositioned volume regions is not significantly disturbed. For example,it may be advantageous to keep secondary maxima of an apodized Besselbeam profile below a threshold intensity needed for nonlinearabsorption.

In view of modifications being subsequent in the feed direction, thegeometry of the modified volume may further be selected such that, for arow of multiple modifications in the feed direction, an earlier inducedmodification has only an insignificant influence on the formation of thefollowing modifications.

As already mentioned, for fast processing, the generation of a singlemodification can be performed with only a single laser pulse/a singlelaser pulse group, so that a position on a work-piece is approached onlyonce in this case.

Ultrashort pulse lasers can make intensities (power densities) availablethat allow causing a sufficiently strong material modification inrespective long interaction zones. The geometric extent of themodification is thereby set with the help of beam shaping such that along extending, high density of free electrons is created by nonlinearabsorption in the material. The supply of energy in deeper regions isperformed laterally, so that the shielding effect by an upstreaminteraction of the plasma can be avoided in comparison to a Gaussianfocusing. For example, an electron density, which extends smoothly inlongitudinal direction, or an electron density, which is modulatedspatially with a high frequency, can be generated.

At the respective intensities, within regions with a sufficiently highdensity of free electrons, an explosive expansion of the material may becaused, whereby the resulting shock-wave can create nanoscopic holes(nano-voids). Additional examples for modifications (modification zones)are changes in the refractive index, compressed and/or tensile stressinduced regions, micro-crystallites, and local changes in stoichiometry.

As explained, by accumulation of such modification zones in feeddirection, a course of a crack can be set. During processing, theworkpiece is accordingly separated along a respective modified contour.The crack formation can then occur directly thereafter or can be inducedby another process. For example, for the separation of non-pre-strainedmaterials, ultrasound ramps or temperature ramps may be used in order tocause a later separation along the modified contour. A singlemodification usually does not lead to crack formation.

With the help of a tailored beam shape, various tension distributionswithin the material and between the modified regions can be created inorder to adapt the separation process to a given material. In theprocess, strong spatial and temporal gradients can favor the formationof a micro- or nano-explosion.

The modification geometry is thereby primarily determined by the beamshaping (and not by the nonlinear propagation as, for example, thefilamentation). The generation of spatial gradients can be achieved bythe optical systems described herein, while the generation of thetemporal gradients can be achieved by pulse trains or pulse shaping.

Generally, a scaling of the intensity distribution of a beam shape canbe achieved by the imaging ratio of the system, in particular by thefocal length and the numerical aperture of the near field optics of theimaging system. Additional possibilities for scaling result from the useof an additional lens as well as the shifting of the beam shapingelement and/or the far field optics (see the description in connectionwith FIGS. 17 and 22). Thus, the lateral and longitudinal extent of thebeam profile within the workpiece can be influenced. In addition,spatial filters and apertures may be used within the beam path for beamshaping, in order to prepare the beam.

Exemplary laser beam parameters for, for example, ultrashort pulse lasersystems and parameters of the optical system and the elongated focalzone, which can be applied within the range of this disclosure, are:

Pulse energy Ep: 1 μJ to 10 mJ (e.g. 20 μJ to 1000 μJ);

Energy of a pulse group Eg: 1 μJ to 10 mJ;

Ranges of wavelength: IR, VIS, UV (e.g. 2 μm>λ>200 nm; e.g. 1550 nm,1064 nm, 1030 nm, 515 nm, 343 nm);

Pulse duration (FWHM): 10 fs to 50 ns (e.g. 200 fs to 20 ns);

Interaction duration (depending on the feed velocity): smaller 100 ns(e.g. 5 ps-15 ns);

Duty cycle (interaction duration to repetition time of the laserpulse/the pulse group): less than or equal to 5%, e.g. less than orequal to 1%;

Raw beam diameter D (1/e2) when entering the optical system: e.g. in therange from 1 mm to 25 mm;

Focal lengths of the near field optics: 3 mm to 100 mm (e.g. 10 mm to 20mm);

Numerical aperture NA of the near field optics: 0.15≤NA≤0.5;

Length of beam profile within the material: larger 20 μm;

Maximal lateral extent of the beam profile within the material, whereapplicable in the short direction: smaller 20λ;

Aspect ratio: larger 20;

Modulation in propagation direction: larger 10 periods over the focuszone;

Feed dv between two neighboring modifications e.g. for separatingapplications:

100 nm<dv<10*lateral extent in feed direction;

Feed during interaction duration: e.g. smaller 5% of the lateral extentin feed direction;

Thus, the pulse duration of the laser pulse and the interaction durationrelate to a temporal range, within which, for example, a group of laserpulses interacts with the material for the formation of a singlemodification at a location. Thereby, the interaction duration is shortregarding the present feed velocity, so that all laser pulses of a groupcontribute to a modification at one position.

If the workpiece is thinner than the focus zone is long, the focus zoneis positioned partially outside of the workpiece, so that modificationsmay be caused that are shorter than the focus zone. Such a situation maybe advantageously used to make the processing process robust also withrespect to varying the distance between the optics and the workpiece. Insome embodiments, a modification may be advantageous that does not reachthrough the complete work-piece. In particular, the length of the focuszone and/or its position within the workpiece may be adapted. In generalit is noted that, due to different thresholds for the nonlinearabsorption, a focus zone with assumed identical intensity may causedifferently large modifications in differing materials.

The aspect ratio relates to the geometry of the beam profile (the focuszone) within the material to be processed as well as the geometry of themodification created with a beam profile. For asymmetric or in lateraldirection modulated (for example, non-rotationally symmetric orring-shaped) beam profiles, the aspect ratio is given by the ratio ofthe length of the modification with respect to a maximum lateral extentin the shortest direction that is present within that range of length.If the beam profile includes a modulation in lateral direction, forexample, for ring-shaped beam profiles, then the aspect ratio relates tothe width of a maximum, for a ring-shaped beam profile, for example, tothe strength of the ring. When multiple modification volumes, which aredisplaced in lateral direction, are formed, the aspect ratio relates tothe lateral extent of a single modification. For a beam profilemodulated in propagation direction (e.g. due to interferences), theaspect ratio relates to the higher ranking total length.

Assuming a distance d between the beam shaping element and the focusinglens (near field optics), which is in particular larger than the focallength fN of the near field optics, and an NA of the near field opticswith respect to air>0.15, the used angular spectrum α of the beamshaping element can be in the range tan(α)<f*NA/d<NA/2 and preferablytan(α)>f*NA/(d*4).

The previously mentioned ranges for parameters may allow the processingof a material thickness up to, for example, 5 mm and more (typically 100μm to 1.1 mm) with roughness of the cutting-edge Ra smaller than, forexample, 1 μm.

Optical system 1 may further include a beam processing unit 13 foradapting beam parameters such as beam diameter, input intensity profile,input divergence, and/or polarization of laser beam 3. For example, thelaser beam of a pulsed laser system is coupled into optical system 1with, for example, a beam diameter of 5 mm, pulse duration of 6 ps atwavelengths around 1030 nm and is led to processing unit 31.

FIG. 3 shows the schematic setup of optical system 1 for explaining thefunctionality. Optical system 1 is based on a beam shaping element 31and an imaging system 33. Beam shaping element 31 is adapted to receivelaser beam 3. Accordingly, it is adapted to a transverse input intensityprofile 41 of laser beam 3. In addition, beam shaping element 31 isadapted to impose onto laser beam 3 a beam shaping phase distribution 43(schematically indicated by dashes in FIG. 1) over transverse inputintensity profile 41. Imposed phase distribution 43 is such that avirtual optical image 53 (essentially) of elongated focus zone 7 isattributed to laser beam 53, with the virtual optical image 53 beinglocated in front of beam shaping element 31. Beam shaping element 31creates in this manner a virtual beam profile that is located upstreamof beam shaping element 31, but does not correspond to the real path ofthe beam being at that position.

Imaging system 33 is constructed such that the virtual beam profile isimaged into the area of the laser processing machine, in which theworkpiece is positioned during the processing. In FIG. 3, imaging system33 includes for that purpose a, in beam direction, first focusingelement, which is referred to herein as far field optics 33A, and a, indirection of the beam, second focusing element, which is referred toherein as near field optics 33B.

Far field optics 33A is provided in the area of phase imposing and isillustrated in FIG. 3 exemplarily by a lens shape downstream of beamshaping element 31. As will be explained in the following, far fieldoptics 33A may also be arranged shortly before beam shaping element 31,composed of components before and after the beam shaping element 31,and/or completely or partially integrated in the beam shaping element31.

After the imposing of the phase within beam shaping element 31, laserbeam 3 propagates in accordance with imaging system 33 over a beamshaping distance Dp to near field optics 33B. Beam shaping distance Dpcorresponds to a propagation length of the laser beam 3, within whichimposed phase distribution 43 transforms the transverse input intensityprofile 41 into a transverse output intensity profile 51 at near fieldoptics 33B. Herein, output intensity profile 51 includes thosetransverse intensity profiles in the optical system that are determinedby the phase imposing. This is usually completed at the latest in thearea of the focal length before the near field optics or within the areaof the near field optics.

For implementing the concept of a virtual beam profile, there are thefollowing considerations for the propagation length (from beam shapingelement 31 to near field optics 33B), which laser beam 3 has topropagate within the optical system. In general, the optical systemforms an imaging system 33 with a far field focusing action and a nearfield focusing action. The latter is determined by near field optics 33Band thereby by near field focal length fN. The former is determined by afar field focusing action and a respective far field focal length fF.Far field focal length fF can be realized by the separate far fieldoptics 33A and/or can be integrated into the beam shaping element. Seein this respect also FIG. 20. Imaging system 33 has an imaging ratio ofX to 1, whereby X for a demagnification of the virtual image usually islarger than 1. For example, imaging ratios are implemented that arelarger than or equal to 1:1 such as larger than or equal to 5:1, 10:1,20:1, or 40:1. In other words, with this definition of the imaging, thefactor X resembles the magnification of the lateral size of the focuszone into the virtual profile. The angle is respectively demagnified.Attention should be paid to the fact that the imaging ratio goesquadratic into the length of the profile. Accordingly, the longitudinallength of a virtual image becomes smaller, for example, for an imagingratio of 10:1 by a factor of 100 and for an imaging ratio of 20:1 by afactor of 400.

At an imaging ratio of 1:1, there is fN=fF, an overlapping alignment ofthe focal planes is assumed. In general, there is fF=X fN. If the farfield optics 33A is integrated into the beam shaping element, it ispositioned, e.g., at a distance fN+fF from the near field optics, i.e.,typically in the range of the sum of the focal lengths of both opticalelements. For a 1:1 or a de-magnifying imaging system, the propagationlength corresponds therefore at least to twice the focal length of thenear field optics.

Separating far field optics 33A and beam shaping element 31 andassuming, that the virtual optical image should not overlap (inparticular not within the intensity region being relevant for the focuszone) with the beam shaping element, the beam shaping element isarranged at at least a distance of I/2 downstream of the longitudinalcenter of virtual beam profile 53. Here, the length I is thelongitudinal extent of virtual beam profile 53 with respect to therelevant intensity area. The longitudinal center of virtual beam profile53 is located, e.g., at the entrance side focal plane of far fieldoptics 33A, which is located at a distance fN+fF from near field optics33B. In this case, the propagation length is d=fN+2fF−I/2=(1+2X)fN−I/2,therefore smaller than fN+2fF=(1+2X) fN, or, in other words, smallerthan the distance between the optical elements plus fF.

For the distance fN+2fF=(1+2X)fN, also for increasing beam enlargementsa respectively increasing length I of virtual beam profile 53 can beimaged, whereby—as explained later—a defined end of the profile can bemaintained.

In general, it is mentioned that, due to raw beam divergences andconvergences as well as for deviating adjustment of the imaging system,deviations from the above considerations may occur. In contrast to acomparable image of a real intensity enhancement, i.e., images withcomparable imaging ratios, the beam shaping element is located closer(see the respective discussion on FIGS. 7 and 8). A common distancetherefore lies in a range (1+2X)fN≥d≥2fN.

Due to the imposed phase, transverse output intensity profile 51includes, in comparison to input intensity profile 41, at least onelocal maximum 49 located outside of a beam axis 45. Local maximum 49being located outside beam axis 45 results in a lateral energy entryinto focus zone 7. Depending on beam shaping element 31, local maximum49 of transverse output intensity profile 51 can be made rotationallysymmetric with respect to beam axis 45—as indicated in FIG. 3 in the cutview—or it can be formed in an azimuthal angular range (see, e.g., FIGS.29 and 30). Usually, the beam axis is defined by the center of thelateral beam profile. The optical system can usually be related to anoptical axis, which usually runs through a symmetry point of the beamshaping element (e.g., through the center of the DOE or the tip of thereflective hollow cone axicon). For rotationally symmetric beams and arespective exact alignment, the beam axis may coincide with the opticalaxis of the optical system at least in sections.

The local maximum can be considered a generic feature of outputintensity profile 51, where in particular for inverse quasi-Bessel beamshapes, a typical substructure with a steep and slowly falling flank canbe formed. That substructure can invert itself due to the focusingaction of the beam forming element and/or the far field optics in therange of an associated far field focal plane. In particular, the outputintensity profile can show within the range of that far field focalplane the local maximum particularly “sharp” or, for example, forinverse quasi-Bessel beam shapes, the local maximum can form itselfquite fast after the beam forming element. However, the aspects of thesubstructure may vary due to the various possibilities in the phaseimposing.

The concept of a virtual beam profile can, on the one side, reduce theconstructional length of optical system 1 and, on the other side, it canavoid the formation of an elongated beam profile with significantintensity enhancement within optical system 1. Imaging system 33 isconfigured such that, within optical system 1, the far field of thevirtual beam profile is formed and that the focusing in the near fieldoptics 33B can be done using a common focusing component such as a lens,a mirror, a microscopic objective, or a combination thereof. In thatcase, “common” is understood herein in the sense of that thecharacteristic beam shape is essentially imposed by beam shaping element31 and not by near field optics 33B.

In FIG. 3, a path of the beam is indicated for illustration thatcorresponds to a beam herein referred to as an inverse quasi-Besselbeam. For that purpose, the path of the beam is illustrated downstreamof beam shaping element 31 with solid lines. Upstream of beam shapingelement 31, instead of incident collimated beam 3, the virtual beamprofile is sketched in analogy to a real quasi-Bessel beam by dashedlines.

Similar to a common quasi-Bessel beam, also the inverse quasi-Besselbeam has a ring structure in the focal plane of far field optics 33A.However, divergent beam areas 55A, 55B indicated in the schematic cutview, which impinge on far field optics 33A, do not result from a “real”quasi-Bessel beam profile, but they result directly from the interactionof beam shaping element 31 with incident laser beam 3. Due to the directinteraction, beam areas 55A, 55B are shaped in their lateral intensitydistribution by transverse beam profile 41 of laser beam 3. Accordingly,for a Gaussian input beam, the intensity decreases in the radialdirection principally in beam areas 55A, 55B away from a beam center.Due to the divergence of beam areas 55A, 55B, typically an area of low(in the ideal case no) intensity is formed accordingly on the beam axisfor the phase-modulated beam portions. In that case, the divergence of abeam portion, accordingly also a divergent beam portion, relates hereinto a beam portion that moves away from the beam axis. However, in thatarea, a beam portion of a phase unmodulated beam and/or also anadditional, phase-modulated beam portion may be superimposed. Withrespect to the development of the beam within the optical system duringthe shaping of an inverse Bessel like beam, it is referred to thedescription of FIGS. 33 and 34. This intensity behavior is schematicallyindicated in transverse intensity courses (e.g., transverse intensitybeam profile segments) 57A and 57B. It is noted that the intensitycourses along the propagation length can change due to imposed phasedistribution 43. At least, however, within the initial area (i.e., beamareas 55A, 55B laying close to beam shaping element 31) and due to thebeam shaping element 31 acting in general as a pure phase mask, theincident intensity profile of laser beam 3 dominates the divergentphase-modulated beam portions.

For a clear explanation of an inverse quasi-Bessel beam, furtherintensity courses 57A′ and 57B′ are schematically indicated in FIG. 3.Here, it is assumed that beam shaping element 31 influences only thephase and not the amplitude. One recognizes that the focusing by farfield optics 33A (or the respective far field action of beam shapingelement 31) reverses the intensity course at the exit of optical system1 such that, during the formation of elongated focus zone 7 on beam axis45, at first low intensities are superposed, which originate from thede-creasing flanks of the incident Gaussian beam profile. Thereafter,the higher intensities superpose, which originate from the central areaof the incident Gaussian beam profile. In this context it is noted thatnot only the intensity on the beam shaping element, but also thecontributing area has to be acknowledged. For rotationally symmetry, thedistance enters accordingly quadratic. As in particular illustrated inconnection with FIG. 4, the longitudinal intensity profile ends exactlyin that area, in which the beam portions from the center of the inputprofile cross. In the center, although the highest intensity is present,the area goes to zero. Moreover, it is noted that, after the focus zone,a reversed intensity course is present again, which corresponds tointensity courses 57A, 57B after the beam shaping element (assuming nointeraction with a material).

Due to imaging with imaging system 33, there are incident virtualintensity courses 57A″ and 57B″, which are accordingly schematicallyindicated with respect to the virtual beam shaping in FIG. 3. Thosecorrespond in principle to intensity courses 57A′ and 57B′.

Those intensity courses, which are inverted in comparison to aquasi-Bessel beam, cause a specific longitudinal intensity course forthe inverse quasi-Bessel beam for focus zone 7 as well as in the virtualbeam profile, i.e., optical image 53, because here the superposition ofbeam portions 55A, 55B is done virtually. For the respective discussionof the intensity course for a conventional quasi-Bessel beam, it isreferred to FIGS. 7 and 8 and the respective description.

FIG. 4 exemplarily illustrates a longitudinal intensity distribution 61within elongated focus zone 7 as it can be calculated for the imaging ofvirtual optical image 53 of an inverse quasi-Bessel beam shape. Depictedis a normed intensity I in Z direction. It is noted that a propagationdirection according to a normal incidence (in Z direction) onto material9 is not required and, as illustrated in connection with FIG. 2, cantake place alternatively under an angle with respect to the Z direction.

One recognizes in FIG. 4 an initially slow intensity increase 61A overseveral 100 micrometer (initial superposition of low (outer)intensities) up to an intensity maximum, followed by a strong intensitydecrease 61B (superposition of the high (central) intensities). For aninverse Bessel beam shape, the result is therefore a hard border of thelongitudinal intensity distribution in propagation direction (the Zdirection in FIG. 4). As one can in particular recognize in view ofintensity courses 57A′ and 57B′ shown in FIG. 3, the hard border isbased on the fact that the end of longitudinal intensity distribution 61relies on the contributions of the beam center of the incident laserbeam having admittedly a lot of intensity, but on a strongly reduced(going to zero) area. In other words, the end relies on the imaging of avirtual beam pro-file in which in the center for the inversequasi-Bessel beam a hole is created. The strong gradient at theintensity decrease at the end relies on the high intensity in the centerof the input profile, limited, however, by the disappearing area. For anideal imaging system, the longitudinal extent of intensity distribution61 is defined by the position of the virtual profile and the imagingscale. If in addition the workpiece includes a large refractive index,the beam profile is accordingly lengthened.

In this context it is added that the hard border has the consequence inlaser processing machines that the, in propagation direction, front endof a modification is essentially stationary in propagation directionalso if the incident transverse beam profile is increased. Themodification changes its extent only in the back part, i.e., it canlengthen in direction to the near field optics, if the input beamdiameter of the laser beam enlarges. A once set position of the hardborder with respect to the workpiece support or the workpiece itself canthereby avoid high intensities downstream of the modification. Incontrast thereto, an enlargement of the input beam diameter, whenimaging a real intensity enhancement, causes an elongation of themodification in propagation direction, i.e., for example into aworkpiece support, which can result in damages of the same.

FIG. 5 shows an exemplary X-Y-cut 63 of the intensity within focus zone7 for the longitudinal intensity distribution 61 shown in FIG. 4. It isnoted that herein some grayscale illustrations such as FIGS. 5, 30, 31are based on a color illustration so that maximum values of theintensity/amplitude can be illustrated dark. For example, the center offocus zone 7 (highest intensity) in FIG. 5 is shown dark and issurrounded by a brighter area of lower intensity. The same applies tofocus zone 707 in FIGS. 30 and 31. One recognizes the elongatedformation of focus zone 7 over several hundred micrometer at atransverse extent of some few micrometer. Together with the thresholdbehavior of the nonlinear absorption, such a beam profile can cause aclearly defined elongated modification within the workpiece. Theelongated shape of focus zone 7 includes, for example, an aspect ratio,i.e., the ratio of the length of the focus zone to a maximal extent inthe lateral shortest direction being present within that length—thelatter for non-rotationally symmetric profiles, in the range from 10:1to 1000:1, e.g. 20:1 or more, for example 50:1 to 400:1.

If one frees oneself from the beam shape—shown in FIG. 4—of an inversequasi-Bessel beam, which is not modified in propagation direction withrespect to amplitude, beam shaping element 31 can additionally create anamplitude redistribution in the far field, which e.g. can be used for anintensity modification in propagation direction. However, the therebycreated intensity distribution in front of focus zone 7 can no longer bepresented in a very clear manner. Nevertheless, often initial stages ofinversions will show up in the beginning region or in the end region ofthe longitudinal intensity profile, for example a slow increase and asteep decrease. However, a (phase caused) amplitude redistribution bythe phase description of beam shaping element 31 may just exactly be setto an inverted intensity distribution, in order to cause, for example, aform of a longitudinal flat top intensity profile.

Additionally, the following feature for distinguishing from a “real”beam shape may be maintained: For the case of a real Gaussian inputbeam, there exists, e.g. for a real axicon, a plane between near fieldoptics and focus zone at which the demagnified Gaussian transverse beamprofile of the input beam is present and can be made visible. Arespective imaging exists for the virtual optical image. However, inthis case, the image plane, in which the demagnified Gaussian transversebeam profile is present, lies behind the focus zone. The transverse beamprofile can accordingly be made visible. This applies generally to phasemasks for the herein disclosed inverse beam shapes, if those areilluminated with a Gaussian beam profile. Specifically, the demagnifiedGaussian transverse beam profile is positioned in the image plane of thebeam shaping element and therefore usually directly downstream of thefocus zone. Due to the already performed divergence, demagnifiedGaussian transverse beam profile is therefore significantly larger thanthe transverse beam profile of the inverse quasi-Bessel beam in thefocus zone. Also, the demagnified Gaussian transverse beam profile ismuch lower in intensity.

One can recognize the position of the imaged Gaussian transverse beamprofile of the input beam by a fast flipping/inversion of the structureof the beam profile, i.e., a strong change over a small lateral area.For example, the transverse intensity profile of the inversequasi-Bessel beam is present in the focus zone. When passing through theimage plane of the beam shaping element, the dark spot in the center isformed “quasi” immediately. For an inverse quasi-Bessel beam, this isdifferent at the beginning of the focus zone. There, due to theincreased superposition of the border areas of the Gaussian beamprofile, a slow transition is made from a dark center to the transverseintensity profile of the inverse quasi-Bessel beam, which is filled inthe center. In other words, in longitudinal direction, the intensityincreases over a larger area then it decreases at the end. At the end,that transition is accordingly clearly sharply limited. It is addedthat, when imaging a real Bessel beam-like intensity enhancement, thebehavior at the end and the behavior at the beginning are interchanged,i.e., at the end of the Bessel beam profile, the dark spot forms moreslowly.

As previously explained, the concept of using a virtual beam profiletherefore has an effect inter alia on the phase imposing to be appliedand the resulting intensity courses in focus zone 7.

FIG. 6 illustrates modification zones 65 that were created in thecontext of an experimental study for examining the formation ofmodifications in a material. Each modification zone 65 goes back to theinteraction with a group of laser pulses, for example two 6 ps pulses ata temporal separation of about 14 ns. The shape of the modificationzones correspond to the shape of elongated focus zone 7 as assumed inaccordance with FIGS. 4 and 5. The maximal length is limited by thegeometry of elongated focus zone 7 at a required intensity/fluence.

The upper four images illustrate the threshold behavior for pulse groupenergies Eg from about 200 μJ to 40 μJ. The lower four images illustratethe shaping of the elongated modification zones 65 at pulse groupenergies Eg from about 30 μJ to 200 μJ. With increasing total energy Eg,the modification zone lengthens in the direction of the beam entrance(near field optics), because the threshold intensity for the nonlinearabsorption is reached within a longer area of focus zone 7. The end ofthe modification in beam propagation direction is in its positionessentially stationary, and even in particular without secondarycorrection of the distance of a near field optics (33B) to the workpieceto be processed. At lower energies, an initial walk in beam direction ofthe back end may occur due to the existing gradient in longitudinaldirection, in particular if the modification threshold lies at smallintensities within the beam profile. However, the walk decreases atmedium and high energies, because the generation of the in-versequasi-Bessel beam profile includes in propagation direction an implicitmaximal back end.

A similar behavior in the change of the longitudinal extent of themodification is also created for a radially increasing beam diameter ofincident laser beam 3. Also in that case, the modification zone islengthening in direction of the beam entrance (near field optics),because the intensity areas of incident laser beam 3, which are added ina radial direction at the outside, guide energy into the longitudinalintensity area in the area of slow intensity increase 61A (i.e.,intensity increase with slow gradient). The maximum of the intensitydistribution will accordingly be shifted in direction of the beamentrance. The end of the modification in beam propagation direction isin contrast in its position essentially stationary, because thatposition is sup-plied with energy by the center of the beam of incidentlaser beam 3. In addition it is noted that this behavior can be observedalso for modified inverse quasi-Bessel beam shapes. For example, for aflat top beam shape as discussed in connection with FIGS. 23 to 26, theposition of the end of the modification would essentially not change fora change in the beam diameter. For such a changed incident intensityprofile, the beam shaping element may further eventually no longerresult in an optimized flat top structure so that this may result inmodulations in the intensity and eventually a variation of thebeginning.

FIG. 7 serves as an illustration of a beam guidance at which a realintensity enhancement 71 is generated by a beam shaping optics 73 suchas an axicon. This corresponds to the known formation of a quasi-Besselbeam. Intensity enhancement 71 is then imaged by a telescope 75 intoworkpiece 9 by forming a focus zone 77. As shown in FIG. 7, in such asetup, there is the danger that the real intensity enhancement 71damages a far field optics 79 of telescope system 75, in particular if asmall setup length is to be realized. The herein disclosed opticalsystem (see, e.g., FIG. 3), which implements the concept of a virtualimage, bypasses that risk of a damage to the beam guiding optics.

FIG. 8 illustrates for completeness a longitudinal intensitydistribution 81 in Z direction that results from the setup of FIG. 7.After an ab initio steep increase 81A, an intensity maximum is reached,beginning at which the intensity decreases again. At lower intensities,a slowly vanishing drop 81B (vanishing drop of small gradient) begins.One sees the general reversal of the longitudinal intensitydistributions 61 and 68 of FIGS. 4 and 8, at which the “hard border” atthe end is replaced by a “hard beginning”.

For such a quasi-Bessel beam, the passing through an axicon with a laserbeam having an incident Gaussian beam profile 83 will result insuperposed beam portions 85A, 85B, the intensity weights of which resultin real longitudinal intensity distribution 81 (at first superpositionof the intensities of the central area of Gaussian beam profile 83, thensuperposition of lower (outer) intensities of Gaussian beam profile 83).For explaining, again schematic intensity courses 87A and 87B areindicated downstream of far field optics 79, and intensity courses 87A′and 87B′ are indicated upstream of focus zone 77.

In the following, various exemplary configurations of optical systemsare explained that implement the concept of virtual intensityenhancement. They comprise beam shaping elements in the transmission andreflection, wherein the imposing of the phase distribution is performedin particularly refractive, reflective, or diffractive. It is referredto the preceding description with respect to the already describedcomponents such as laser system 11.

In view of the distances of beam shaping optics 73 from the near fieldoptics, the following values can apply similar to the considerations forthe virtual image. For a real beam profile, one would typically positionthe center of the to be imaged real beam profile of length I in theentrance-side focal length of the far field optics. A typical distancewould then be at least

fN+2fF+I/2=(1+2X)fN+I/2, thus larger than fN+2fF, in other words, largerthan the distance between the optical elements plus fF.

FIG. 9 shows a refractive beam shaping with the help of a hollow coneaxicon 131A. This creates a virtual inverse quasi-Bessel beam profile153A upward of hollow cone axicon 131A. The same is indicated in FIG. 9by dashed lines, a real intensity enhancement is not present in thatarea. In addition, in the embodiment of FIG. 9, the far field optics isconfigured in beam propagation direction downstream of hollow coneaxicon 131A as plano-convex lens 133A. Near field optics 33B causes thefocusing of the laser beam into focus zone 7, so that the virtualinverse quasi-Bessel beam profile 153A is related to the laser beam asvirtual optical image of focus zone 7.

FIG. 10 shows an embodiment with a hollow cone axicon lens system 131Bthat is used as a refractive beam shaping element. Here, the far fieldoptics is integrated in the beam shaping element as convex lens surface133B, which is positioned at the entrance side of the hollow coneaxicon. This setup creates similarly a virtual inverse quasi-Bessel beamprofile 153B.

FIG. 11A illustrates an embodiment with a reflective beam shapingelement, in particular a reflective axicon mirror system 131C. A highlyreflective surface of the beam shaping element is shaped such that thebeam shaping feature of a reflective axicon is combined with the farfield forming component of a focusing hollow mirror. Accordingly, axiconmirror system 131C has the function of beam shaping as well as of thefar field optics. A virtual inverse quasi-Bessel beam profile 153C isindicated at the backside of axicon mirror system 131C, thus in an areathat is not passed by laser beam 3.

As is further shown in FIG. 11A, after beam adaptation unit 13, laserbeam 3 of laser system 11 is coupled into optical system 1 by adeflection mirror 140. Deflection mirror 140 is, for example, arrangedon the optical axis between axicon mirror system 131C and near fieldoptics 33B and guides the beam to beam shaping element 131C. In someembodiments, the deflection mirror may, for example, be centrallydrilled through to guide as less as possible light onto the central areaof beam shaping element 131C, which potentially has optical errors. Inaddition to those aspects of filtering described in the following inconnection with FIGS. 17 and 18, it is already noted at this stage thatdeflection mirror 140 at the same time blocks an undesired central beamportion such that the same is not focused by near field optics 33B.

FIG. 11B shows a further embodiment of an optical system based on areflective beam shaping element. The beam shaping element in form ofreflective axicon mirror system 131C is illuminated thereby with laserbeam 3 through an opening 141 of a drilled through deflection mirror140′. That reflected and phase imposed beam impinges then after theformation of a e.g. ring-shaped far field onto deflection mirror 140′.The same guides the beam onto near field optics 33B for focusing intothe elongated focus zone. The opening serves accordingly in addition askind of a filter/diaphragm of the central area of the reflected beam.

In another embodiment with a reflective beam shaping element, theoptical system includes a reflective axicon, a drilled throughoff-axis-parabolic mirror, and the near field optics. That reflectiveaxicon includes for the beam shaping a conical grinded based body, theconical surface of which is coated highly reflective. The laser beam canbe irradiated through the opening in the off-axis-parabolic mirror ontothe reflective axicon. The reflected and beam shaped beam impinges thenon the off-axis-parabolic mirror that redirects the beam on near fieldoptics 33B and at the same time collimates the same.

FIGS. 12 and 13 show embodiments of the optical system with digitalizedbeam shaping elements. Here, the digitalization can relate to the use ofdiscrete values for the phase shift and/or the lateral structure (forexample, pixel structure). The use of spatial light modulators (SLMs) isone of many different possibilities to realize the beam shaping withprogrammable or also permanently written diffractive optical elements(DOE).

In addition to the simple generation of one or more virtual beamprofiles, e.g. according to the phase imposing of one or more hollowcone axicons, diffractive optical elements allow the desiredmodification, for example, for homogenizing of the longitudinalintensity distribution. For this, deviations in the phase canexemplarily be used in the range equal to or smaller than 50%, e.g.equal to or smaller than 20% or equal to or smaller than 10% withrespect to, for ex-ample, the hollow cone axicon phase (and thereby ofan inverse quasi-Bessel beam). In general, SLMs allow very fine phasechanges at a lateral rough resolution, in contrast to, for example,lithographically generated, permanently written DOEs. Permanentlywritten DOEs comprise e.g. plano-parallel steps, the thickness of whichdetermine the phase. So, the lithographic manufacturing allows a largelateral resolution. Binary steps can result in real and virtual beampro-files. Only a number of more than two phase steps can result in adifferentiation in the sense of a preferred direction for the virtualbeam profile. For example, four or eight or more phase steps allow anefficient beam shaping with respect to the virtual beam profile.However, the discretization can cause secondary orders that can, forexample, be filtered out. In general, several optical elements can becombined within a DOE, by determining e.g. the transmission function ofall elements (e.g. hollow cone axicon(s) and lens(es); adding theindividual phase functions (exp(−1i(phi1+phi2+ . . . )). In addition oralternatively, some type of superposition of individual transmissionfunctions can be done. For the determination of the phase distributions,it was initially referred to the publication of Leach et al.Manufacturing methods for continuous microstructures comprise, forexample, the analog-lithography or the nanoimprint-lithography.

Herein, the structural element of a diffractive optical beam shapingelement, which causes the phase imposing and is configured in an arealshape, be it an adjustable SLM or a permanently written DOE, is referredto as a phase mask. Depending on the type of configuration of the DOE,it may be used in transmission or in reflection to impose a phasedistribution on a laser beam.

In FIG. 12, a spatial light modulator 31A is used in reflection forphase imposing. For example, spatial light modulator 31A is based on a“liquid crystal on silicon” (LCOS) that enables a phase shift that isprogrammable for the individual pixels. Spatial light modulators canfurther be based on micro-systems (MEMS), micro-opto-electro-mechanicalsystems (MOEMS), or micro-mirror-matrix systems. In SLMs, the pixelscan, for example, be controlled electronically to cause a specific phaseimposing over the transverse input intensity profile. The electronicalcontrollability allows, for example, the online-setting of phases and,thus, the adaptation of focus zone 7, e.g. in dependence of the materialto be processed or in reaction of fluctuations of the laser. In theconfiguration of FIG. 12, the function of a diffractive axicon for thegeneration of a virtual inverse quasi-Bessel beam profile can becombined, for example, with the far field forming action of a far fieldoptics by the phase shifting of the spatial light modulator 31A.Alternatively, a permanently written reflective DOE can be used as beamshaping element 31A.

FIG. 13 is a schematic view of an optical system based on a DOE 31B, forwhich the phase imposing is permanently written in DOE 31B. DOE 31B isused in that case in transmission. As in FIG. 12, the phase shift,which, for example, results in a virtual quasi-Bessel beam profile, aswell as the focusing property of far field optics are combined withinthe DOE 31B.

The optical systems of FIGS. 9 to 13 can result in output intensityprofiles that correspond to inverse quasi-Bessel beam profiles and thatare attributed to virtual optical images.

FIG. 14 illustrates an example of a phase distribution 243 as it can beprovided e.g. in DOE 31B. Phase distribution 243 is rotationallysymmetric. One recognizes ring-shaped phase distributions, the frequencyof which is modulated in radial direction. The rings point to thegeneration of a rotationally symmetric virtual quasi-Bessel beamprofile. The frequency modulation points to the integration of the phasecomponent of the far field optics in the phase distribution for beamshaping. In FIG. 14, the phases are indicated in the range of ±π. Inalternative configurations, discrete phase distributions such as binaryphase distributions or multi-step (for example, 4 or more levels in therange of the phase shift from 0 to 2π) phase distributions can beimplemented in DOE phase masks.

FIGS. 15 and 16 illustrate exemplarily an output intensity profile 251within the intensity cross-section (FIG. 15) and in the 2D-top view(FIG. 16). One recognizes an intensity maximum 249 extending in a ringshape around beam axis 45. There is hardly any intensity in the beamcenter.

In some embodiments, the transition into the inverse quasi-Bessel beamwill not be complete such that accordingly a non-phase-modulatedremaining beam, for example with a Gaussian beam shape, is superposed tothe ring-shaped intensity profile. FIG. 15 indicates schematically sucha non-phase-modulated beam portion 252 by a dash-dotted line.

Maximum 249 of that intensity distribution in FIG. 15 is an example of alocal intensity maximum, with which an original input intensity profile(e.g. a Gaussian beam profile) was modified in the area of thetransverse output intensity profile. The rotational symmetry of the ringstructure is due to the rotational symmetry of the inverse quasi-Besselbeam profile. In alternative embodiments, the local intensity maximum islimited to an azimuthal angular range. In addition, a superposition ofazimuthal limited and/or ring-shaped local maxima may be given.

When using a refractive hollow cone axicon (see FIGS. 9 and 10) for thegeneration of an inverse quasi-Bessel beam-shaped output intensityprofile, undesired beam portions may be created under undesired anglesfor a non-perfect tip of the axicon. Also for diffractive beam shapingelements, non-desired beam portions may appear. For example, anon-phase-modulated beam portion, which cannot be neglected, oradditional orders of diffraction in the far field of the laser beam canbe present.

The herein disclosed optical systems simplify, by using the far fieldcomponents, the insertion and the shape selection of filters to filterout such disturbing beam portions. In particular these undesired beamportions can be separated from the desired beam portions (beam for use)in a simple manner in the area of the Fourier plane.

Referring to the non-phase-modulated beam portion 252 of FIG. 15, FIG.17 shows an exemplary optical system that is based on the optical systemshown in FIG. 3. However, additionally a filtering ofnon-phase-modulated portions is performed in the area of the Fourierplane of imaging system 33. As an example, a spatial filter unit 220 ispositioned upstream of near field optics 33B in FIG. 17.

Filter unit 220 includes a central area around beam axis 45 that blocks,for example, the Gaussian intensity distribution—indicated in FIG. 15—ofthe non-phase-modulated beam portion 252. Filter unit 220 canadditionally include sections, which are located radially further awayfrom the beam axis 45, for blocking higher orders of diffraction by theDOE or the SLM.

In general, filter unit 220 is provided for the suppression ofnon-phase-modulated base modes and higher diffraction orders as well asof scattered radiation of the various herein disclosed refractive,reflective, or diffractive beam shaping elements. For rotationallysymmetric output intensity profiles, usually also the filter unit ismade rotationally symmetric. In some embodiments, only some portions offilter unit 220 or no filtering at all is provided.

Diffractive beam shaping elements allow a further approach forsuppressing the non-phase-modulated beam portions. For this, anadditional phase contribution is imposed to deflect the phase-modulatedbeam portion.

FIG. 18 shows, for example, an optical system in which the diffractiveoptical element 31 is additionally provided with a linear phasecontribution. The linear phase contribution results in a deflection 230of phase-modulated beam 203A. Non-phase-modulated beam portion 203B isnot deflected and impinges, for example, on a filter unit 222.

FIG. 19 shows a further embodiment of an optical system that utilizesthe use of the far field component additionally for the implementationof a scan approach. In general, a scan system allows shifting focus zone7 within a certain range. In general, it is possible by the separationof the beam shape from the near field focusing to provide favorabletelecentric scan approaches, in particular for the volume absorption. Insome embodiments, in addition the location as well as the angle can beset. Accordingly, such scanner systems can allow writing fine contoursinto a workpiece.

In the configuration of FIG. 19, a scanner mirror 310 is positioned atthe image side focal plane of a near field optics 333B. Scanner mirror310 deflects the laser beam in the range of the out-put intensitydistribution onto near field optics 333B positioned at the side. Thedeflection in the Fourier plane results in that the propagationdirection in the workpiece is preserved despite the displacement inlocation. The scanning region itself is determined by the size of nearfield optics 333B.

If scanner mirror 310 is not correctly positioned in the focal plane ofnear field optics 333B or if it can be moved with respect thereto, thenan orientation of the elongated focus zone, in particular an angulardeviation from the Z direction in FIG. 2, can be set.

With the help of a configuration in accordance with the optical systemshown in FIG. 13, FIG. 20 explains exemplarily the underlying imagingfeatures. The optical system includes a beam shaping element 31 thatoperates also as a far field optics and is therefore characterized by afocal length fF. In addition, the optical system includes near fieldoptics 33B that is characterized by focal length fN. In FIG. 20, thefocal planes of the far field optics and the near field optics coincide.Accordingly, in FIG. 20 only one focal plane 340 is indicated by adashed line. In that configuration of overlapping focal planes, theimaging system images for incidence of a plane wave front generally avirtual beam shape 253 onto elongated focus zone 7, for example, aninverse quasi-Bessel beam profile, inverse modulated or homogenizedquasi-Bessel beam profiles as examples for inverse quasi-Bessel/Airybeam shapes.

Though the focal planes do not need to overlap always. For example, theimaging system can be adapted to a given beam divergence, but laser beam3 may be incident with another divergence. In those cases, still avirtual optical image being positioned in front of the beam shapingelement is attributed to elongated focus zone 7, but it does not need tobe a perfect imaging. A similar situation may be given for an intendedmisalignment of the imaging system, for example, in connection with ascanner device.

FIG. 20 illustrates in addition the terms “far field optics” and “nearfield optics”. The far field optics creates the far field of virtualbeam path 253 in the range of far field focal length fF. As previouslyalready explained, the far field optics can be distributed in itsfunction, for example, be made of one or more components, which arearranged before and/or after the beam shaping element and displaced withrespect to the same, and/or be integrated into the beam shaping element.The near field optics focuses the beam with the smaller focal length fNin the direction of the workpiece and thereby forms the focus zone.Thus, the far field of virtual beam profile 53 with respect to the farfield optics, as well as the far field of focus zone 7 with respect tonear field optics 33B is positioned in the area of focal plane 340.

Also for non-perfect imaging (e.g. non-overlapping focus planes of farfield optics and near field optics), essentially an acceptable intensitydistribution in the focus zone can be given, because the intensityprofile, which impinges onto the near field optics, changes only alittle.

For example, in the case of an inverse quasi-Bessel beam shape, thefirst focusing by the far field optics within the optical system causesan adaptation of the ring size on the near field optics. In that manner,the far field optics has a focusing action onto the ring diameter,which, as indicated in the figures, decreases up to some type ofintermediate focus.

FIG. 21 illustrates the beam path in an optical system for the case thata convergent laser beam 3′ impinges on beam shaping element 31.Phase-modulated portion 303A of the laser beam is focused onto elongatedfocus zone 7. Due to the convergence of incident laser beam 3′ (andeventually due to a separate focusing far field optics or an integrationinto the phase distribution of beam shaping element 31), thenon-phase-modulated portion 303B (dash dotted line) decreases furtherduring the propagation length Dp and impinges on a central area of nearfield optics 33B. Thereby, a focus 350 for non-phase-modulated beamportion 303B is formed that is closer to near field lens 33B than it iselongated focus zone 7. The non-phase-modulated portion is stronglydivergent after focus 350, so that those intensities are no longerreached within the workpiece with respect to the non-phase-modulatedbeam portion 303B that result in nonlinear absorption. In such aconfiguration, one can do without filtering non-phase-modulated beamportions 303B.

Nevertheless, a spatially localized filter unit can be provided in thearea of focus 350 (or even between far field optics and near fieldoptics, if the beam is strongly focused) such that non-phase-modulatedbeam portion 303B is kept out of the interaction zone and the workpiece.

FIG. 22 shows an optical system that is equipped with an additional lens400 upstream of beam shaping element 31. Lens 400—as an example of anadditional focusing component—is located at a distance DA to beamshaping element 31.

Beam shaping element 31 has a phase distribution that is set for aspecific beam diameter. The illuminated part of that beam shapingelement, i.e. the beam diameter of the input intensity profile at beamshaping element 31, can be adapted by the translatability of lens 400with respect to beam shaping unit 31.

In some embodiments, lens 400 can be compensated before beam shapingelement 31 within the phase mask of beam shaping element 31 so that theimaging does not change and only the 0th order, i.e. thenon-phase-modulated, portion is focused.

In general, lens 400 can also be understood as a component of the farfield optics. If the far field optics includes a plurality ofcomponents, which can be translated with respect to each other and withrespect to the near field optics, then the imaging scale can be changedby a suitable translation. In some embodiments, lens 400, the beamshaping element, or both can be translated together to adjust theimaging scale of optical system 1. In some embodiments, lens 400 can beused as a first telescope-part-lens for adapting the beam diameter onthe beam shaping element, whereby a second telescope-part-lens iscalculated into the phase mask.

In some embodiments, lens 400 can be translated to perform a fineadjustment of the raw beam in particular for a longitudinal flat topbeam shape or multi-spot formation.

If the input beam is selected such that a convergent or divergent beamis present at beam shaping element 31, then one can—also in this case inaccordance with FIG. 21 under certain conditions—do not use a filterunit for non-phase-modulated beam portion 403B. I.e., intensities forthe nonlinear absorption within the workpiece are only reached by thephase-modulated beam portion 403A.

Diffractive optical elements allow a digitalized and e.g. pixel basedphase adaptation over the input intensity profile. Starting from theintensity distribution of an inverse quasi-Bessel beam shape, alongitudinal flat top intensity profile can, for example, be generatedin focus zone 7. For that purpose, the phase distribution within thebeam shaping element can be influenced such that intensity contributionsin the output intensity profile are taken out of the area, which formsthe intensity maximum and the tails of the Bessel beam, and are radiallyredistributed by a phase change such that, for the later focusing bynear field optics 33B, the increasing area 61A and the decreasing area61B are magnified or far extending tails are avoided to the most part(e.g. by pushing power from the tails into the homogenized area).

A respective output intensity profile 551 is shown in FIG. 23 (intensitycross-section) and FIG. 24 (2D-top view). One recognizes in theintensity cross-section of FIG. 23 that—in comparison to FIG. 15—thelocal maximum is broadened in the radial direction and modulated. Theresult is a respectively radially extended modulated ring structure 549.

FIG. 25 shows the focusing of such an output intensity distribution 551.The result is a longitudinal quasi-homogenized intensity distribution(flat top) 561 over a range from about 700 μm in Z direction.

In analogy to FIG. 6, FIG. 26 shows modification zones 565(modifications) in a transparent material 9. The upper four imagesillustrate again the threshold behavior for pulse group energies Eg fromabout 20 μJ to 40 μJ, while the lower four images show increasing pulsegroup energies Eg from about 30 μJ to 200 μJ. One recognizes that, whenthe threshold is passed, the modification zones form essentially alwaysover the same range of extent in Z direction within workpiece 9. This isbased on the essentially constant intensity having only a short increaseand drop off. With increasing energy, however, not only the strength butalso the lateral extent of the modification zones increases.

Another embodiment is shown in FIG. 27, which allows reaching a sequenceof intensity enhancement in propagation direction. In general,supplemental phase imposing can be done in the area of the image sidefocal plane of near field optics 33B such as lateral and/or longitudinalmulti-spot phase imposing. Specifically, one recognizes in FIG. 27 asequence of three intensity maxima 661A, 661B, and 661C, which each havean intensity distribution in accordance with FIG. 4.

This sequence can be generated by a longitudinal multi-spot phaseimposing or the use of a multi-focal lens as near field optics 33B. So,for example, an additional diffractive optical element may be providedin the area of the Fourier plane (focal plane of near field optics 33B)or close to near field optics 33B, which provides an additionalphase-modulation for the three foci. Such phase adaptations are known,for example, from EP 1 212 166 B1, incorporated herein by reference inits entirety.

In connection with FIGS. 28 to 31, a further potential formation of anelongated focus zone is illustrated for the case of an accelerated Airybeam shape.

FIG. 28 shows a phase distribution 743 as it can be imposed within beamshaping element 31 onto the input intensity profile. Here, facedistribution 743 includes the phase distribution, which is required fora generation of the accelerated beam, and the phase distribution of aconcave lens, which compensates a raw beam convergence. In general, aphase mask of an accelerated beam creates a well collimated beam whichdoes not change significantly over the propagation distance and which isthen focused with the near field component in a so-called acceleratedbeam shape.

FIGS. 29 and 30 illustrate the associated output intensity profile 751in the cut view (FIG. 29) and in the top view (FIG. 30). One recognizesthat the intensity maximum is displaced slightly from the center (i.e.beside the beam axis 45) in Y direction. Thus, the transverse outputintensity profile 751 is modified with respect to the input intensityprofile with a local maximum 749, which is located outside of beam axis45.

The focusing of such an output intensity profile 751 results inelongated and curved focus zone 707 that is illustrated in FIG. 31.Thereby it is allowed that such an accelerated beam pro-file can be usedalso in combination with non-transparent media, if the focus zone isguided, for example, in Y direction to the border of such a material.The resulting interaction would, for example, provide a rounding of theside of the material. In other embodiments, such a beam profile can beused with transparent materials for cutting with curved cutting faces.

In some embodiments, an optical system is configured, for example, suchthat a real intensity enhancement in accordance with FIG. 7 as well as avirtual intensity enhancement in accordance with FIG. 3 is created.Thereby, the longitudinal extent of modification zones can be widened.

FIG. 32 shows schematically an exemplary optical system with a binaryDOE 31C. If a laser beam 3 falls onto binary DOE 31C, on the one hand, areal intensity enhancement 871 is formed, for example, a quasi-Besselbeam downstream of DOE 31C. On the other hand, a beam portion is formed,which is associated with a virtual image 853—downstream of DOE 31C—of anelongated focus zone 807A, for example, in the shape of an inversequasi-Bessel beam.

The optical system includes further a telescope system 833 with a farfield optics 833A and a near field optics 833B. Telescope system 833images virtual image 853 as well as real intensity enhancement 871 intothe material 9 to be processed. For that purpose, binary DOE 31C ispositioned in or close to the focal plane of far field optics 833A.

The imaging results in an enlarged interaction region that includeselongated focus zone 807A and focus zone 807B that originates from thereal intensity enhancement 871. In the resulting sequence of successivefocus zones 807A and 807B, the intensity for (inverse) quasi-Besselbeams is at first in accordance with the intensity distribution shown inFIG. 4 and there-after in accordance with the intensity distributionshown in FIG. 8. The result is an intensity distribution with a lowintensity intermediate space that is formed by the strong intensity drop61B and the strong intensity raise 81A. That low intensity intermediatespace can, for example, be provided in the region of a contact zone whenprocessing a pair of on each other lying workpieces. In addition, thisapproach allows that one can achieve twice the length for theinteraction for identical input beam diameter and identical angularrange, which is covered by the optical system.

In some embodiments, the non-phase-modulated portion can be focused inthe area between the successive focus zones 807A and 807B. A respectiveGaussian focus 807C is additionally shown schematically in FIG. 32. Insuch an embodiment, an adaptation of the efficiency of the diffractionmay become possible, because the non-phase-modulated beam may be usedfor filling intensity voids.

Herein, some aspects were described exemplarily based on selectedvirtual beam profiles. In general, those aspects can be transferred ontothe herein as (inverse) virtual beam shapes described types of beamssuch as inverse quasi-Bessel/Airy beam shapes, e.g. inverse quasi-Besselbeam profiles or inverse modulated or homogenized quasi-Bessel beamprofiles.

In connection with FIGS. 33A to 33D and 34, the propagation from thebeam shaping element to the near field optics is explained by beamprofiles and amplitude courses for an in-verse quasi-Bessel beam.Lighter grayscale values correspond to larger amplitudes. A respectiveinverted quasi-Bessel beam can be generated with the herein disclosedrefractive, reflective, and diffractive optical systems, for example,with the hollow cone axicon systems and the DOE systems. A DOE systemcan be based, for example, on the phase distribution of a phase maskshown in FIG. 14, in which a focusing phase contribution is provided inaddition to the phase required for the inverse quasi-Bessel beam.

It is assumed that a laser beam having a rotationally symmetric Gaussianbeam profile is irradiated onto the beam shaping element. A Gaussianbeam profile includes a transverse amplitude course that runs throughthe beam center in a Gaussian manner. The FIGS. 33A, 33B, 33C, and 33Dshow respectively the development of the beam profiles 900A, 900B, 900C,and 900D and the respective schematic amplitude courses 902A, 902B,902C, and 902D, the latter directly after the beam shaping element atz=0 mm and at a distance downstream at z=10 mm, z=50 mm as well as inthe focal plane of the successive near field component at z=200 mm. Atransition of 100% is assumed, i.e., one does not generate a strayradiation portion e.g. in terms of non-phase-modulated or scatteredlight.

FIG. 34 shows the amplitude distribution for a step along the beam axisZ beginning at the exit of the beam shaping element at z=0 up to thenear field lens at z=250 mm. The positions of the beam profiles 900A,900B, 900C, and 900D are indicated in FIG. 34 with arrows.

One recognizes that, due to the pure phase mask, a Gaussian beam profile900A and a Gaussian amplitude course 902A are still present directlyafter the beam shaping element similar to the Gaussian beam. A sharplylimited hole is then immediately formed, however, caused by the imposedphase, which yields the additional divergence. Already at z=10 mm, onerecognizes a clear dark spot 904 in the center of the beam profile 900B.The same is continuously growing. At the same time, a ring area 906 withhigher amplitude is formed.

Ring area 906 is sharply limited towards the inside, which can be seenat a step shape in the radial amplitude/intensity distribution. A flank907 of the circumferential step faces towards that beam axis/towards thebeam center. With increasing z values, the opposing sections of flank907 get separated, i.e. the central sharply limited hole grows fast indiameter (D1<D2).

In the radial amplitude/intensity distribution, ring area 906 dropstowards the outside with increasing z values faster and faster. Thisdevelopment is schematically shown in the falling flanks 908A to 908C ofthe amplitude courses 902A to 902C. In the far field, i.e., for examplein the overlapping focal planes of the imposed focusing (far field)action and the near field optics, a sharp ring 908D is formed withinbeam profile 900D, that thereafter diverges (see FIG. 34). Thereby, nowa sharp edge is performed at the outer side, i.e., the step of the innerflank now faces towards the outside.

In FIG. 34, one recognizes the sharp edge in the transition between darkarea 910A, which broadens in Z direction, and border area 910B, whichnarrows in Z direction and is more bright, whereby the grayscale valuesin brighter border area 910B at first have higher values radially insideand then, beginning at the focal plane, have higher values radiallyoutside.

This general behavior of the beam profile and the amplitude coursesenable a test of an optical system with a Gaussian input beam, for whichat first a hole forms with a steep flank facing the inside and therebyresults in a local maximum outside of the beam axis in the far field. Animaging of the beam profile from the inner area as well as in the areaof the focus zone can identify the respective beam profile. The use ofthe optical system is thereby not necessarily limited to Gaussian beams.In addition, it is to note that the figures are a result of calculationsfor the ideal case. For example, if a non-ideal DOE is used, theaddressed non-phase-modulated portion for higher orders or a portion ofa real quasi-Bessel beam (such as for a binary mask) can be on the beamaxis and can fill the “hole” with intensity.

An inverse quasi-Bessel beam can therefore comprise a step with a steepflank in the amplitude course and accordingly in the intensitydistribution. The same can in particular face to the in-side in the areaclose to the beam shaping element, for example, in the area up to halfof the far field, and in particular in the area of a focus length of thefar field optics downstream of the beam shaping element. For a “simple”inverse quasi-Bessel beam without base at the beam axis, theamplitude/intensity increases in the range of the step from almost zeroto the maximum of the phase-modulated beam portion. Thereby, theformation of the step (within the phase-modulated beam portion) is alsogiven for an exemplary incident beam having essentially a constantradial intensity (radial flat top) across the beam shaping element,because the step concerns essentially the beam center.

The beam characteristic described before upstream of the far field focalplane is thereafter radially inverted up to the focus zone. After thatfocus zone, it inverts radially another time such that again a stepshape can be formed at that position—without interaction with a materialto be processed. The beam profile can, for example, be analyzed bytaking the beam at a respective position, be it within the opticalsystem after the beam shaping element or before or after the focus zone.In particular for setups, which allow a blocking of a central disturbingbeam, one can analyze the intensity distribution of the phase-modulatedbeam portion before or after the focus area.

In this context, it is further referred to the German patent applicationfiled by the same applicant at the same day that in particular discussespossibilities of using DOEs when generating inverse quasi-Besselbeam-like or inverse quasi-Airy beam shapes. The content of thatapplication is herein incorporated in its completeness. As is explainedtherein generally, for example, several steps can be formed whengenerating several (inverse) quasi-Bessel beams, which in the case ofthe relation to a virtual image can comprise, strongly pronounced flanksthat show in the respective longitudinal sections to the inside (beforefar field focal plane and after focus zone) or to the outside (betweenfar field focal plane and focus zone).

Further embodiments and/or further developments of the herein disclosedaspects are summarized in the following:

The transverse output profile can correspond to a far field intensityprofile of the virtual optical image and/or of a far field intensityprofile of the focus zone with respect to the near field optics.

A given input beam shape of the laser beam can comprise the transverseinput intensity profile, a beam diameter, a transverse input phaseprofile, and input divergence, and/or a polarization, and the opticalsystem can be configured such that the given input beam shape istransformed into a convergent output beam shape at the exit of the nearfield optics, whereby the near field of the output beam shape forms theelongated focus zone.

The optical system can comprise a supplementing phase imposing unit inthe area of the image side focal plane of the near field optics, inparticular for the lateral and/or longitudinal multi-spot phaseimposing.

In general, the herein disclosed focusing elements such as the far fieldoptics and the near field optics can be configured as, for example,lens, mirror, DOE, or a combination thereof.

Moreover, additional optical elements can be inserted into opticalsystems. Among others, intermediate images can be inserted in theimaging system, to realize, for example, a filter function as well as ascan movement in the area of the image-side focal plane. Thereby, e.g.,the image-side focal plane (e.g. image plane 340 in FIG. 20) can itselfbe imaged by an additional optical system. Alternatively oradditionally, such optical intermediate systems can allow, for example,an enlarged working distance and/or a magnification of the working fieldfor scanner application.

It is explicitly stated that all features disclosed in the descriptionand/or the claims are intended to be disclosed separately andindependently from each other for the purpose of original disclosure aswell as for the purpose of restricting the claimed inventionindependently of the composition of the features in the embodimentsand/or the claims. It is explicitly stated that all value ranges orindications of groups of entities disclose every possible intermediatevalue or intermediate entity for the purpose of original disclosure aswell as for the purpose of restricting the claimed invention, inparticular as limits of value ranges.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. An optical system for beam shaping a laser beamfor processing a transparent material by modifying the transparentmaterial in a focus zone that is elongated in propagation direction, theoptical system comprising: a beam shaping element arranged to receivethe laser beam having a transverse input intensity profile and isconfigured to impose a beam shaping phase distribution over thetransverse input intensity profile onto the laser beam; and one or morenear field optical elements arranged downstream of the beam shapingelement at a beam shaping distance (Dp) and configured to focus thelaser beam into the focus zone, wherein the imposed beam shaping phasedistribution is such that a virtual optical image of the elongated focuszone is attributed to the laser beam, the virtual optical image beinglocated before the beam shaping element, and the beam shaping distance(Dp) corresponds to a propagation length of the laser beam within whichthe imposed phase distribution transforms the transverse input intensityprofile into a transverse output intensity profile in the region of theone or more near field optical elements, and the transverse outputintensity profile has, in comparison with the input intensity profile, alocal maximum lying outside of a beam axis.
 2. The optical system ofclaim 1, wherein the optical system is configured as an imaging systemwith a demagnifying imaging ratio for imaging the virtual optical imageand for generating the elongated focus zone.
 3. The optical system ofclaim 2, wherein the beam shaping element is as a part of the imagingsystem further configured to impose onto the laser beam a sphericalphase distribution with focusing action such that the imaging of thevirtual optical image onto the elongated focus zone is performed by theadditionally imposed phase distribution of the beam shaping element andthe focusing by the one or more near field optical elements.
 4. Theoptical system of claim 2, wherein the optical system comprises one ormore far field optical elements with focusing action, which is arrangedclose to the beam shaping element such that the imaging of the virtualoptical image onto the elongated focus zone is performed by the focusingwith the one or more far field optical elements and the one or more nearfield optical elements.
 5. The optical system of claim 1, wherein theimaging system attributes to the beam shaping element an image planedownstream of a longitudinal center of the image of the virtual opticalimage, and a transverse beam profile of the laser beam is present at thebeam shaping element in the image plane.
 6. The optical system of claim5, wherein there is in the region of the image plane a change, whichchanges fast in longitudinal direction, from a lateral beam profile,which is given in the focus zone, to a lateral beam profile having adark center, the latter for an lateral Gaussian beam profile of thelaser beam and with respect to beam portions of the incident laser beam,which generate a divergent beam area that is attributed to the virtualoptical image.
 7. The optical system of claim 1, wherein the opticalsystem is configured such that only a central area of the incident laserbeam contributes to a downstream end of the focus zone attributed to thevirtual image, so that a change of the beam diameter of the incidentlaser beam does not result in an longitudinal displacement of thedownstream end of the focus zone.
 8. The optical system of claim 1,wherein the imposed beam shaping phase distribution is configured togenerate, in the case of an incident laser beam having a Gaussianintensity distribution, at least for a portion of the incident laserbeam a divergent beam area, which is attributed to the virtual opticalimage and comprises downstream of the diffractive optical beam shapingelement a transverse intensity distribution, which decreases from theinside to the outside.
 9. The optical system of claim 4, wherein thetransverse intensity distribution is present upstream of a downstreamfocal plane of the one or more far field optical elements, and a phaseimposed beam area comprises a lateral intensity distribution having asection of a step-shaped increase of intensity, which comprises aradially inward facing flank in the region between the beam shapingelement and a focal plane, which is attributed to at least one of theone or more near field optical elements, the one or more far fieldoptical elements and the phase with focusing action providedadditionally to the beam shaping element.
 10. The optical system ofclaim 9, wherein the lateral intensity distribution comprises a radiallyoutward facing flank in the region between the focal plane and the focuszone.
 11. The optical system of claim 9, wherein the phase distributionis such that the focusing of such a phase imposed beam portion forms atleast one of an inverse quasi-Bessel beam-like beam profile or aninverse quasi-Airy beam-like beam profile having a focus zone beingelongated in propagation direction, for which only a central area of theincident laser beam provides contributions to a downstream end of theelongated focus zone.
 12. The optical system of claim 1, wherein thebeam shaping element is configured as only phase-modulating so that whenimposing the phase distribution the input intensity profile at the beamshaping element is maintained, while an amplitude modulation is avoided.13. The optical system of claim 1, wherein the beam shaping elementcomprises a hollow cone axicon-lens or mirror system, a reflectiveaxicon-lens or mirror system, or a programmable diffractive opticalelement.
 14. The optical system of claim 1, wherein the beam shapingelement is further configured to impose a linear phase distribution, sothat a spatial separation of a usable beam portion from a disturbingbeam portion is achieved by a lateral beam deflection of the usable beamportion.
 15. The optical system of claim 1, wherein the elongated focuszone comprises an aspect ratio of at least 10:1.
 16. The optical systemof claim 1, wherein the elongated focus zone is a focus zone of aninverse Bessel beam-like beam or of an inverse Airy beam-like beam or acombination therefore.
 17. The optical system of claim 1, furthercomprising a scan unit for scanning the elongated focus zone withrespect to the material.
 18. The optical system of claim 1, furthercomprising a beam preparation unit for adapting at least one of theinput intensity profile, the input divergence, and the polarization ofthe laser beam.
 19. A laser processing machine for processing amaterial, which is transparent for the laser beam, with a laser beam bymodifying the transparent material in a focus zone, which is elongatedin propagation direction of the laser beam, the machine comprising: alaser beam source; an optical system according to claim 1; and aworkpiece positioning unit for positioning the transparent material asworkpiece.
 20. The laser processing machine of claim 19, furtherconfigured to perform a relative movement between the workpiecepositioning unit and the focus zone, the laser processing machinefurther comprising a control unit configured perform a control operationselected from the group of control operations consisting of: setting adownstream end of the elongated focus zone, with respect to theworkpiece positioning unit; and setting a parameter of the laser beamand the optical system, which causes an elongation of the elongatedfocus zone in the direction upstream, wherein at the same time theposition of the downstream end of the elongated focus zone is maintainedwith respect to the workpiece positioning unit without a follow-upcorrection of a distance of one or more near field optical elements tothe workpiece positioning unit.
 21. The laser processing machine ofclaim 19, wherein the laser beam source is further configured togenerate a laser beam that modifies the transparent material bynonlinear absorption.
 22. The laser processing machine of claim 21,wherein the laser beam source is further configured to focus laserpulses to a fluence of 2 J/cm2 within the elongated focus zone.
 23. Anoptical system for beam shaping a laser beam for processing atransparent material by modifying the transparent material, the opticalsystem comprising: a beam shaping element for imposing a phasedistribution of at least one of an inverse quasi-Bessel beam-like beamprofile and of an inverse quasi-Airy beam-like beam profile onto thelaser beam; and one or more near field optical elements for focusing thephase imposed beam, wherein the phase distribution is such that thefocusing of the phase imposed beam forms an inverse quasi-Besselbeam-like beam profile or an inverse quasi-Airy beam-like beam profilewith a focus zone elongated in propagation direction of the laser beam,for which only a central region of the incident laser beam makescontributions to a downstream end of the elongated focus zone.
 24. Amethod for beam shaping a laser beam with a transverse input intensityprofile for processing a transparent material in a focus zone, which iselongated in propagation direction, the method comprising: imposing abeam shaping phase distribution onto the transverse input intensityprofile, wherein the imposed phase distribution is such that a virtualoptical image of the elongated focus zone is attributed to the laserbeam; propagating the laser beam over a beam shaping distance (Dp),after which the imposed phase distribution has transferred thetransverse input intensity profile into a transverse output intensityprofile, so that the transverse output intensity profile, in comparisonto the input intensity profile, comprises a local maximum locatedoutside of the beam axis; and focusing the laser beam into the focuszone for forming a near field, which it is based on the output intensityprofile.
 25. The method of claim 24, wherein imposing of the phasedistribution onto the transverse input intensity profile is performedtogether with at least one step selected from the group of stepsconsisting of: imposing a spherical phase distribution onto the laserbeam; imposing a linear phase distribution onto the laser beam;filtering out a beam portion of the laser beam; filtering out a centralnon-modulated beam portion of the laser beam; and filtering out beamportions of higher diffraction order from the laser beam.
 26. A methodfor laser material processing of a transparent material with a laserbeam, the method comprising: generating at least one of an inversequasi-Bessel beam-like laser beam profile or an inverse quasi-Airybeam-like laser beam profile with a focus zone, which is elongated inpropagation direction, by phase-modulation of the laser beam by shapingthe laser beam with an optical system, wherein the optical systemincludes: a beam shaping element arranged to receive the laser beamhaving a transverse input intensity profile and is configured to imposea beam shaping phase distribution over the transverse input intensityprofile onto the laser beam, and one or more near field optical elementsarranged downstream of the beam shaping element at a beam shapingdistance (Dp) and configured to focus the laser beam into the focuszone, wherein the imposed beam shaping phase distribution is such that avirtual optical image of the elongated focus zone is attributed to thelaser beam, the virtual optical image being located before the beamshaping element, and the beam shaping distance (Dp) corresponds to apropagation length of the laser beam within which the imposed phasedistribution transforms the transverse input intensity profile into atransverse output intensity profile in the region of the one or morenear field optical elements, and the transverse output intensity profilehas, in comparison with the input intensity profile, a local maximumlying outside of a beam axis; positioning the elongated focus zone atleast partly in the material to be processed; and setting a parameter ofthe laser beam, which extends the elongated focus zone in directionupstream, wherein the position of the downstream end of the elongatedfocus zone is maintained with respect to the workpiece positioning unitwithout a follow up correction of a distance of the one or more nearfield optical elements to the workpiece positioning unit.
 27. The methodof claim 26, further comprising: setting a position of a downstream endof the elongated focus zone, with respect to a workpiece positioningunit.